The present application is a national stage application of PCT Pat. App. No. PCT/US2012/027952, filed Mar. 7, 2012, which claims priority to U.S. Provisional Patent Application having Ser. No. 61/451,326, which was filed Mar. 10, 2011. These priority applications are incorporated by reference in their entirety into the present application, to the extent that these priority applications are not inconsistent with the present application.
Compressor unloader valves, also known as “unloaders,” are often used in reciprocating compressors to optimize compressor efficiency at various throughput rates. Generally, a reciprocating compressor includes a piston, which moves back-and-forth in a chamber. The piston has a compression stroke, during which the piston compresses a process gas between itself and an end of the chamber, and a return stroke, during which the piston is drawn back to bottom dead center to begin the next compression stroke. Reciprocating compressors also include an inlet line through which the process gas to be compressed is received into the chamber, and a discharge line through which the process gas is expelled after compression. A discharge valve and a suction valve are positioned in the suction and discharge positions of the cylinder, respectively. The discharge and suction valves are typically check valves; thus, in normal operation, the discharge valve allows compressed gas to exit the compressor chamber, but prohibits it from flowing back into the chamber. Similarly, the suction valve allows the process gas into the chamber, but prevents it from flowing back out.
To control the amount of gas compressed, and thus provide optimum compression for a given throughput, unloaders are often used. One type of unloader operates by holding the suction valve elements of the suction valve open after the piston arrives at bottom dead center and begins the compression stroke. Thus, instead of being compressed, the process gas is allowed to flow past the open suction valve elements and back into the inlet line until the desired amount of gas to be compressed for the stroke remains in the cylinder. The unloader then releases the suction valve elements, allowing them to close.
Accordingly, infinite step controllers (ISCs) may be used to provide variable valve opening duration during the return and/or compression stroke. One type of ISC employs fingers, which are attached to an actuator piston. The fingers engage the suction valve elements, preventing them from closing. The actuator piston movement is controlled by a hydraulics system to quickly achieve optimum unloader position. Such hydraulics, while proven reliable and acceptable in many applications, increase the complexity of the ISC system. Accordingly, such hydraulically-actuated ISCs can lead to increased operating and maintenance costs. What is needed, therefore, is a system and method for providing a reliable ISC that is capable of moving rapidly and with precision to provide a range of suction valve positions.